Manufacture of levobupivacaine and analogues thereof from L-lysine

ABSTRACT

The subject invention concerns a compound of formula (X), ##STR1## which can be prepared from the starting material L-lysine, and converted to levobupivacaine by cyclization.

This application is a 371, filed Oct. 9, 1995.

FIELD OF THE INVENTION

This invention relates to a cost-effective process for the conversion,via diazotisation methodology, of L-lysine to a carboxanilide precursorof levobupivacaine and related compounds.

BACKGROUND OF THE INVENTION

Compounds of formula 1 wherein R is a methyl, n-propyl or n-butyl groupare widely used as local anaesthetics. Biological studies have shownthat the (S)-enantiomers of such compounds (e.g. levobupivacaine whereinR is n-butyl) display lower cardiotoxicity than the correspondingracemates whilst maintaining the same anaesthetic potency, and aretherefore more beneficial for clinical uses. Thus there is a requirementfor efficient processes to manufacture compounds of formula 1 in theform of single enantiomers. The process embodied by the presentinvention employs a chirality pool approach to levobupivacaine,commencing from L-lysine, an inexpensive starting material which isreadily available in bulk.

Although L-lysine has been converted through to optically enrichedL-pipecolic acid and esters thereof by diazotisation and cyclisationreactions, cyclisation of an intermediate to form apiperidine-2-carboxanilide directly has hitherto not been reported.Furthermore, in the context of the present invention, these existingmethods are hampered by an excessive number of steps required formanipulation of protecting groups in order to obviate Walden inversionat the carboxyl-bearing centre, which leads to the formation ofD-pipecolic acid.

An additional benefit of the invention is the provision of unnaturalamino acids of formula 2 with (S) configuration, which are important aspharmaceutical intermediates, e.g. for incorporation intophysiologically active synthetic peptides. Compound 2a is commonlyprepared by a multistep synthesis of the racemate commencing fromdihydropyran followed by enzymatic resolution of an ester derivative(e.g. p-nitrobenzyl). This process in inefficient with respect to thenumber of steps required and by the fact that resolution produces up to50% of an unwanted enantiomer. Diazotisation of L-lysine and derivativesthereof has been identified as an alternative strategy for thepreparation of 2-amino-6-hydroxyhexanoic acid and derivatives thereof.However, such an approach has hitherto only been applied to thesynthesis of compound 2c, but this process, employing sodiumnitroprusside, is low yielding (28%).

Copper complexation of lysine and similar amino-acids has beenextensively used as a technique for temporary protection of the α-aminoacid moiety, as disclosed by Ledger et al, Anstr. J. Chem. 18:933-5(1965). This procedure facilitates selective transformation of thesecond amino group present in the side-chain, e.g. the attachment of acovalently-bonded amino-protecting group.

SUMMARY OF THE INVENTION

This invention provdes a cost-effective process for preparing compoundsof formula 1, wherein R is H or alkyl, from the inexpensive startingmaterial L-lysine, using a chirality pool approach, and involvingscission of the terminal C--N bond of L-lysine. In particular, thepresent invention provides a practical and economical process for themanufacture of levobupivacaine from L-lysine, which is exemplified inScheme 1. This process involves the preparation of novel compounds offormula (X) as defined in claim 1; either R' is a protecting group, inwhich case R is H in the product, which can then be alkylated asdesired, or R' is alkyl and corresponds to R as alkyl.

DESCRIPTION OF THE INVENTION

Formula (X), and other formulae used herein, refer to compounds thathave at least predominantly the given stereochemistry. They are at leastsubstantially free of their optical antipode, e.g. in at least 50%, morepreferably at least 70% or 99%, enantiomeric excess.

According to the particular reactions shown in Scheme 1, theα-N-benzyloxycarbonyl derivative of L-lysine is initially (a) subjectedto diazotisation in the presence of acetic acid to afford(S)-2-(benzyloxycarbonylamino)-6-acetoxyhexanoic acid (2b).Alternatively, Z may be any other blocking group, many examples of whichare well known to the skilled man.

The next three steps of Scheme 1 proceed by way of novel compounds 3-5:(b) Condensation with 2,6-dimethylaniline gives compound 3; (c) cleavageof the 0-acetyl group by methanolysis gives compound 4; and (d)tosylation gives compound 5. The OTs group shown in Scheme 1 is merelyone example of many leaving groups which can be used and which are knownto the skilled man. Examples are given in claim 2.

Finally, compound 5 is (a) subjected to hydrogenation in the presence ofbase to effect direct conversion to enantiomerically pure(S)2',6'-dimethylpiperidine-2-carboxanilide 6. Variants of this processinclude conversion of the α-N-benzyloxycarbonyl (or other blocked)derivative of L-lysine to compound 6 via alkyl halide derivatives offormula 7. The conversion of compound 6 to levobupivacaine or otherN-alkylated analogues can be carried out by methodology known to thoseskilled in the art, or as described in another Patent Application filedtoday, claiming priority from British Patent Application No. 9421478.0.Preferably, R is methyl, n-propyl, cyclopropyl or, most preferably,n-butyl.

A preferred aspect of the process summarised in Scheme 1 is the use ofeither sodium nitrite/sodium acetate/acetic acid or isoamylnitrite/acetic acid as efficient reagent systems for the conversion ofα-N-benzyloxycarbonyl or otherwise blocked L-lysine to the compound 2b,by formation of a diazonium intermediate followed by solvolysis of thisspecies or of the carbocation formed by loss of N₂. This discoveryprovides significant benefit over the prior art process since the yieldsare much higher (60-87% of 2b compared to 28% 2c).

As a further feature of the invention, we have discovered that L-lysinecan be cleanly and efficiently converted to(S)-2-amino-6-hydroxyhexanoic acid 2a in a process (Scheme 2) involvingsequential formation of bis(lysinato)copper, diazotisation-solvolysis togenerate the novel copper(II) complex 8, and cation-exchange to effectdecomplexation. Diazotisation-solvolysis in such processes can beeffected in aqueous media using either sodium nitroprusside at pH>10 orsodium nitrite, e.g. at pH<4.

More generally, NH₂ may be converted to any leaving group Y, e.g.halide, usually under anhydrous conditions, using known methodology.Depending on the stability of the metal complex, the reaction proceedsin one or more steps, to give a compound of the formula

    Y--(CH.sub.2).sub.4 --CH(NH.sub.2)--COOH

in optically-enriched form. The free amine may be blocked at this stage,as a reactant for use in Scheme 1.

Such processes are advantageous over conventional diazotisations ofL-lysine, which give mixtures of products arising from unselectivetransformation of either one, and in some cases both, of the two aminegroups present.

The following Examples illustrate the invention. Examples 1 to 5illustrate respective steps in Scheme 1; Example 6 illustrates Scheme 2.

EXAMPLE 1

A stirred solution of N.sup.α -benzyloxycarbonyl L-lysine (6.33 g, 22.6mmol) in acetic acid (150 ml) was treated with sodium acetate (1.85 g)and then sodium nitrite (1.56 g×3) at 40° C. The reaction was stirredfor 3 hours at 40° C., cooled and the bulk of the acetic acid wasremoved in vacuo and the residues partitioned between water (100 ml) anddichloromethane (100 ml×3). The combined organic extracts were dried(MgSO₄) and evaporated in vacuo to give compound 2b a yellow oil (6.35g, 87%).

EXAMPLE 2

Isoamyl nitrite (6.1 ml) was added to a stirred solution of N.sup.α-benzyloxycarbonyl L-lysine (6.34 g, 22.6 mmol) in glacial acetic acid(35 ml) at 22° C. The mixture was heated at 50° C. for 12 hours, thenvolatile material removed by distillation in vacuo (last traces removedvia formation of azeotopic mixtures with toluene) to give compound 2b(4.38 g, 60%).

EXAMPLE 3

A solution of compound 2b (6.35g, 19.7 mmol) in dichloromethane (150 ml)was treated sequentially with 2,6-dimethylaniline and a solution ofdicyclohexylcarbodiimide (4.89 g) in dichloromethane (10 ml) at 18° C.The reaction was stirred for 24 hours. The precipitate which formed wasfiltered and washed with dichloromethane (100 ml). The organics wereconcentrated in vacuo to give compound 3 as waxy solid. Without furthertreatment, this material was dissolved in MeOH (150 ml) and solid K₂ CO₃(9.57 g) was added. The reaction was stirred for 24 hours then filteredand evaporated to leave a waxy solid which was chromatographed on silicagel with 1.5:1 EtOAc:heptane to give compound 4 as a colourless solid(2.22 g, 28%).

EXAMPLE 4

p-Toluenesulphonyl chloride (0.4 g) was added to a solution of compound4 (0.60 g, 1.7 mmol) and pyridine (1.4 ml) in dichloromethane (5 ml) andstirred for 4 hours. Dilute hydrochloric acid (1 N; 10 ml) was added,and after stirring for 18 hours the mixture was extracted withdichloromethane (50 ml). The organic solution was washed with aqueousNaOH (1 N; 10 ml), dried (MgSO₄) and evaporated in vacuo to give ayellow oil which was chromatographed on silica gel eluting with 1:1EtOAc:heptane. This gave compound 5 as a colourless oil (0.76 g, 81%)which slowly solidifies as the tosyl derivative.

EXAMPLE 5

A mixture of compound 5 (0.76 g, 1.4 mmol), Pd/C (0.076 g) and K₂ CO₃(0.47 g) in EtOH (15 ml) was placed under an atmosphere of hydrogen(balloon) and stirred vigourously for 3 hours. The reaction was filteredand the solids thoroughly washed with EtOH (5 ml). The combined filtrateand washings were concentrated in vacuo and the residue partitionedbetween dichloromethane (3×25 ml) and aqueous NaOH (1 N; 25 ml). Thecombined organic extracts were dried (MgSO₄) and concentrated in vacuoto give (S)-2',6'-dimethylpiperidine-2-carboxanilide 6 as a colourlesssolid (0.284g, 87%), which was shown by chiral HPLC analysis to have anoptical purity of >98%.

EXAMPLE 6

A suspension of L-lysine (0.931 g, 5.1 mmol) and CuCO₃.Cu(OH)₂ (1.24 g,5.6 mmol) in water (15 ml) was heated under reflux for 5 minutes thencooled and filtered. The pH of the filtrate was adjusted to 4 with 2M H₂SO₄. A solution of sodium nitrite (0.70 g, 10 mmol) in water (5 ml) wasadded dropwise over 10 minutes and the mixture was strirred at 24° C.for 6 hours. Chelex 100 resin (40 g) was added to effect decomplexation.The resulting suspension was then filtered to give a solution ofL-lysine (60%) and (S)-2-amino-6-hydroxyhexanoic acid 2 (30%). ##STR2##

I claim:
 1. A process for the manufacture of levobupivacaine, whereinsaid process comprises reaction of a compound of formula (X), wherein R'is butyl or a removable amino-protecting group, under conditions wherebythe NHR' group cyclises to the CH₂ Y group, with loss of the leavinggroup Y; and, if R' is an N-protecting group, N-butylation of thecyclised product.
 2. A process for the manufacture of ropivacaine,wherein said process comprises reaction of a compound of formula (X),##STR3## wherein R' is propyl, or a removable amino protecting group,under conditions whereby the NHR' group cyclises to the CH₂ Y group,with loss of the leaving group Y; and, if R' is an N-protecting group,N-propylation of the cyclised product.
 3. The process according to claim1, wherein said conditions comprise hydrogenation in the presence ofbase.
 4. The process according to claim 1, which additionally comprisesthe prior steps of diazotisation-solvolysis of α-N-blocked-L-lysine;condensation of the resultant (S)-2-(blocked amino)-6-acetoxyhexanoicacid with 2,6-dimethylaniline; cleavage of the o-acetyl group, bymethanolysis; and introducing the group Y into the resultant compound,to give the compound (X).
 5. The process according to claim 1, whichadditionally comprises the prior steps of reacting L-lysine with Cu²⁺ion; subjecting the resultant complex to diazotisation-solvolysis;cation-exchange to effect decomplexation; condensation of the resultant(S)-2-amino-6-hydrohexanoic acid with 2,6-dimethylaniline; andintroducing the group Y and the blocking group R' into the resultantcompound, to give the compound (X).
 6. The process according to claim 2,wherein said conditions comprise hydrogenation in the presence of base.7. The process according to claim 2, which additionally comprises theprior steps of diazotisation-solvolysis of α-N-blocked-L-lysine;condensation of the resultant (S)-2-(blocked amino)-6-acetoxyhexanoicacid with 2,6-dimethylaniline; cleavage of the o-acetyl group, bymethanolysis; and introducing the group Y into the resultant compound,to give the compound (X).
 8. The process according to claim 2, whichadditionally comprises the prior steps of reacting L-lysine with Cu²⁺ion; subjecting the resultant complex to diazotisation-solvolysis;cation-exchange to effect decomplexation; condensation of the resultant(S)-2-amino-6-hydrohexanoic acid with 2,6-dimethylaniline; andintroducing the group Y and the blocking group R' into the resultantcompound, to give the compound (X).
 9. The process according to claim 1,wherein R' of formula (X) is an amino-protecting group.
 10. The processaccording to claim 1, wherein Y of formula (X) is selected from thegroup consisting of N₂, OH, O-acyl, O-alkylsulphonyl, O-arylsulphonyland halide.
 11. The process according to claim 1, wherein R' of formula(X) is benzyloxycarbonyl.
 12. The process according to claim 1, whereinY is p-toluenesulphonyloxy.
 13. The process according to claim 5,wherein diazotisation-solvolysis is effected using sodium nitroprussideat basic pH.
 14. The process according to claim 5, whereindiazotisation-solvolysis is effected using sodium nitrite at acidic pH.15. The process according to claim 13, wherein said pH is above about10.
 16. The process according to claim 14, wherein said pH is belowabout
 5. 17. The process according to claim 8, whereindiazotisation-solvolysis is effected using sodium nitroprusside at basicpH.
 18. The process according to claim 8, whereindiazotisation-solvolysis is effected using sodium nitrite at acidic pH.19. The process according to claim 17, wherein said pH is above about10.
 20. The process according to claim 18, wherein said pH is belowabout
 5. 21. The process, according to claim 4, wherein thediazotisation/solvolysis comprises using sodium nitrite and sodiumacetate in acetic acid.
 22. The process according to claim 4, whereinthe diazotisation/solvolysis comprises using isoamyl nitrite in aceticacid.
 23. The process according to claim 7, wherein thediazotisation/solvolysis comprises using sodium nitrite and sodiumacetate in acetic acid.
 24. The process according to claim 7, whereinthe diazotisation/solvolysis comprises using isoamyl nitrite in aceticacid.
 25. The process according to claim 2, wherein R' of formula (X) isan amino-protecting group.
 26. The process according to claim 2, whereinY of formula (X) is selected from the group consisting of N₂, OII,O-acyl, O-alkylsulphonyl, O-arylsulphonyl and halide.
 27. The processaccording to claim 2, wherein R' of formula (X) is benzyloxycarbonyl.28. The process according to claim 27, wherein Y isp-toluenesulphonyloxy.